Ethylene treatment by gas permeation

ABSTRACT

The invention relates to a process for separating off ethylene from a gas mixture comprising more than 8% by volume of carbon dioxide or oxygen, by gas permeation.

This is a divisional application of U.S. application Ser. No. 09/529,520filed Jul. 27, 2000.

The invention relates to a process for preparing carbonyl compounds fromolefins and an oxidizing agent in which the olefin is oxidized in areactor, a reaction gas mixture which comprises unoxidized olefin beingformed.

The invention relates In particular to a process for preparingacetaldehyde by the Wacker-Hoechst process (EP 0 006 523 A1,Incorporated into the present application by reference) from the rawmaterials ethylene and oxygen In the presence of an aqueous catalystsolution (copper(II) chloride and palladium chloride).

The raw materials ethylene and oxygen react, on an industrial scale, ina vertically upright reactor which is filled with the catalyst solution.The ethylene is fed to the reaction by a continuously recirculated gasstream (recycle gas). The gas stream leaving the reactor, which gasstream, In addition to the reaction product acetaldehyde, comprisesstream, unreacted ethylene, carbon dioxide (CO₂), oxygen (O₂) and smallamounts of minor components (reaction gas mixture), is conducted offfrom the reaction compartment and treated by a multistage process.

Water and aldehyde are separated off by a 2-stage condensation (waterseparation and gas cooler) with subsequent scrubbing. The aldehyde-freerecycle gas which, In the standard operating state, can comprise 60-80%by volume ethylene, 3-7% by volume oxygen and 8-25% by volume CO₂ asmain components, is compressed to compensate for the loss of pressure,admixed with fresh ethylene and fed to the reactor.

To keep the concentrations of ethylene and inert constituents such asCO₂, nitrogen, argon, methane and ethane constant in the recyle gas, itis expedient to bleed off a certain gas stream constantly from thesystem. Because of it considerable ethylene content, it is advantageousto make further use of the gas stream as raw material (world marketprice of ethylene currently approximately 800-DM/metric ton), For thispurpose, it is first cooled to reduce the moisture content, thencompressed using compressors and after passing through an absorptiondryer, Is transmitted via a pipeline to a consumer. If this pathway ofoffgas utilization is Impossible, the offgas must be flared off via ahigh flare.

Alternatively, there are essentially two possibilities for utilization;

a) thermal utilization and

b) treatment by absorption or adsorption, for example of CO₂ in sodiumhydroxide solution, and recycling the ethylene to the process.

Ethene regeneration integrated into the process is initially to bepreferred to thermal utilization of the offgas. CO₂ and O₂ could inprinciple be separated off by an absorption process, but this would behighly complex because of the high CO₂ and O₂ contents and wouldintervene in the licensed material cycle.

The object underlying the invention therefore was to improve the processmentioned at the outset by a suitable separation process.

It has now surprisingly been found that this object can be achieved byseparating off the unoxidized olefin, ethylene in the specific case,from the reaction gas mixture by the membrane separation process of gaspermeation.

Whereas gas permeation In the case of separation of N2O₂, for example,belongs to the prior art, its potential use in the present case issurprising in that the selectivity of commercially available membranesfor the component ethylene (C₂H₄) is generally classified asinsufficient for a quantitative separation with acceptable yields.Ethylene has a great similarity to methane (CH₄), which is enriched bygas permeation industrially (e.g. in natural gas or landfill gas) onlyto 60% by volume methane, because the loss of methane is excessive athigher concentrations.

The invention therefore relates to a process for preparing carbonylcompounds from olefins and an oxidizing agent in which the olefin isoxidized in a reactor, a reaction gas mixture which comprises unoxidizedolefin being formed, which comprises separating off unoxidized olefin atleast in part from the reaction gas mixture by a membrane process.

The invention therefore likewise relates to a process for separating offa gaseous olefin from a gas mixture comprising more than 8% by volumecarbon dioxide or oxygen, which comprises separating off the olefin fromthe gas mixture by gas permeation.

In a first particular embodiment, the carbonyl compound is acetaldhydeand/or the olefin is ethene and/or the oxidizing agent is oxygen. Theunoxidized olefin can here either diffuse through the membrane or beretained by it. Possible membranes are polymer membranes, preferablypolyimide membranes. However, suitable membranes are also membranes ofinorganic materials, preferably of ceramic or metal, palladium orplatinum being particularly preferred. In further particularembodiments, the absolute pressure on the reaction gas mixture side isin the range from 1 to 80 bar abs., preferably in the range from 3 to 40bar abs., or the absolute pressure on the permeate side is less than orequal to 2 bar abs., preferably less than or equal to 1 bar abs.,particularly preferably less than or equal to 200 mbar abs. The membraneprocess is preferably carried out at temperatures in the range from 0 to100° C., preferably In the range from 10 to 40° C. The membranes arepreferably present in the form of spirally wound modules or hollow fibermodules.

Particular embodiments are given by the features of the subclaims. Oneor more of these features can also represent, together or each alone,solutions according to the invention of the object and these featurescan also be combined In any manner.

An exemplary embodiment of the process according to the invention isdescribed in more detail below with reference to the Wacker-Hoechetprocess mentioned at the outset and to the FIGS. 1 to 8. No restrictionof the invention in any manner is intended by this.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures:

FIG. 1: shows a diagrammatic representation of the principle of gasseparation using membranes;

FIG. 2: shows a diagrammatic representation of the separation mechanismsin gas separation using membranes,

FIG. 3: shows a diagrammatic representation of the structure of gaspermeation membranes;

FIG. 4: shows a diagrammatic representation of the structure of hollowfiber modules;

FIG. 5: shows a local separation characteristic;

FIG. 6: shows a process flow chart of an exemplary embodiment of theprocess according to the invention;

FIG. 7: shows a presentation of the results of a calculation ofseparation efficiencies for a single-stage membrane separation;

FIG. 8: shows a presentation of the results of a calculation ofseparation efficiencies for a two-stage membrane separation;

FIG. 1 illustrates gas separation using membranes: a gas stream (feed)is passed to a membrane by a transport device, for example by means of acompressor. Some of the gas permeates through the membrane and is takenoff as permeate; the residual part whose composition has changed owingto the selective permeation of gas components, is passed on asconcentrate.

As is seen in FIG. 2, a distinction with respect to the separationmechanism is made between gas separation through pore-free, microporousand porous membranes.

Owing to the low selectivity of porous membranes, enrichments are onlyachievable here by multistage cascades.

With the development of what are termed asymmetric membranes (based onthe pore structure, see FIG. 2), whose separating action is based on thesolution-diffusion mechanism, the process has also been able toestablish itself on an industrial scale for some classes of separationproblems.

The separation mechanism (mass transport in membranes) may be derived,as is known, from integrating the generalized formulation of Fick's lawfor the flux J $\begin{matrix}{J = {{- c_{kM}} \cdot \frac{D_{{kM},0}}{RT} \cdot \frac{\partial\mu_{kM}}{\partial z}}} & (1)\end{matrix}$

over the solution-diffusion layer with the preconditions:

a) no coupling between the permeate fluxes,

b) equality of the chemical potential between the exterior and membranephase on both sides of the active layer (μ_(k)=μ_(k,m)) and the linearrelationship

J=Q _(k)·(x _(k) p _(F) −y _(k) p _(P))

the variables and constants having the customary meanings In thiscontext. Accordingly, the molar flux of each permeating component isproportional to the difference between the partial pressures of thiscomponent on both sides of the membrane. The permeability Q_(k) relatedto the membrane thickness is a parameter specific to substance andmembrane which must be determined by experiment. It is generallyproportional to the diffusivity D_(k) and to the solubility S_(k) ofcomponent k in the membrane and Inversely proportional to the membranethickness δ:${Q_{k} - {\frac{D_{k} \cdot S_{k}}{\delta}\quad {where}\quad S_{k}}} = \frac{c_{kM}}{P_{k}}$

It may be concluded from the remarks on mass transport insolution-diffusion membranes that efficient membranes are distinguishedby the fact that the product of solubility and diffusivity in themembrane polymer should be as large as possible for one component of amixture and as small as possible for the other component(s) of themixture to be separated. Furthermore, the thickness δ of thesolution-diffusion layer should be as small as possible.

For economic use of gas permeation, the fluxes through symmetricalmembranes are much too small, even if separation units (modules) havinga high packing density are used. The gas permeation has therefore,similarly to reverse osmosis, not been of interest until the moment whenit was possible to increase the permeate flux through asymmetricmembranes (FIG. 3. variant A) by a factor of 100 with approximately thesame selectivity.

With respect to permeability, the pore-free polymer membranes of eachgroup in FIG. 3 are at least qualitatively similar: thus the membranesin the glass state all have high permeabilities for water vapor, helium,hydrogen and carbon monoxide, but in contrast they are less permeable tonitrogen and methane and ethylene. Solution-dffusion membranes ofrubber-like polymers, in contrast, are all distinguished by highpermeabilities for organic solvents in comparison with permanent gasessuch as O₂, N₂l and are therefore suitable for separating off solventsfrom exhaust air, for example.

In gas permeation, use is principally made of hollow fiber and capillarymodules. In hollow fiber and capillary modules, the membranes arepresent in the form of very thin pressure-stable tubes. They are eithercombined in parallel axially or, if they are true hollow fibers, in theshape of a helix, also. In this case, there exist, in accordance withFIG. 4, both modules in which the crude mixture flows on the exterior ofthe fibers, and modules in which the crude mixture flows In the fibers.

As explained, local mass transport in the separation of mixtures ofpermanent gases by pore-free polymer membranes can in many cases bedescribed with sufficient accuracy by equation 2. For a binary mixture,the local permeate composition (local separation characteristics) can becalculated in an idealized manner from the local high-pressure sideretentate concentration, feed pressure and the permeate pressure, aswell as the permeabilities of the membrane. Since binary mixtures arespecified by stating one concentration, only the more rapidly permeatingcomponent is considered below and the index “i” is omitted (x° x_(i), y°y_(i)), Using equation 2 for both components, on the basis of the massbalance and material balance, after conversion,$y^{\prime} = {{{\frac{1}{2} \cdot \left\lbrack {1 + {\delta \cdot \left( {x + \frac{1}{\alpha - 1}} \right)}} \right\rbrack} - \sqrt{\left\lbrack {\frac{1}{2} \cdot \left\lbrack {1 + {\delta \cdot \left( {x + \frac{1}{\alpha - 1}} \right)}} \right\rbrack} \right\rbrack^{2} - \frac{\alpha \cdot \delta \cdot x}{\alpha - 1}}} = {y\left( {x,\alpha,\delta} \right)}}$

${{- {ideal}}\quad {separation}\quad {factor}\quad \alpha} = {\frac{Q_{i}}{Q_{j}} \geq 1}$${{where}\quad {pressure}\quad {ratio}\quad \delta} = {\frac{Pp}{Pp} \geq 15}$

The plot of the equation (3) is shown in FIG. 5.

It can be seen from the diagram that the separation characteristicsimprove with increasing pressure ratio δ and with increasing separationfactor α. Furthermore it is clear that a membrane acts selectively forthe more rapidly permeating component over the entire concentrationrange. This applies even it this component is only present in traces(ppm range), although then the possible accumulation is restricted.

The separation characteristics, in addition to the ideal separationfactor α, are also restricted by the pressure ratio δ. Unrestrictedconcentration cannot be achieved even with an ideally selectivemembrane.

Table 1 shows the mean composition of the ejected ethylene offgas of alarge industrial plant over half a year, and the rounded means as basevalues.

It can be clearly seen that the components ethylene, carbon dioxide,oxygen, argon and nitrogen represent the majority of the gas mixture.The ethylene workup therefore essentially requires separating off CO₂and O₂ from the offgas (argon and N₂ have lower contents). Thisseparation step can be performed according to the invention by using themembrane process of gas permeation.

TABLE 1 Mean composition of the ethylene offgas Component Mean valuesBase values Oxygen O₂ 5.84 6 Argon Ar 3 3 Nitrogen N₂ 1.03 1 Methane CH₄0.53 0,5 Carbon dioxide CO₂ 11.58 12 Ethene C₂ H₄ 76.84 77 Ethane C₂ H₆0.69 0 Methyl chloride CH₃Cl 0.62 0,5 Vinyl chloride 0.01 0 Acetaldehyde0.01 0 Ethyl chloride 0.09 0 Furan 0.04 0 Chloroform 0.0067 0

FIG. 6 shows the flow diagram of an acetaldehyde production processaccording to the invention, extended by the gas permeation, theindividual reference numbers having the following meanings: 1 reactor, 2mist eliminator, 3 titanium cooler, 4 gas cooler, 5 gas scrubber, 6recycle gas fan, 7 contact stripping column, 8 regenerator, 9 vaporseparator, 10 vapor cooler, 11 vapor scrubber, 12 crude aldehyde vessel,13 heat exchanger, 14 degassing column, 15 purification column, 16condensation, 17 pure aldehyde reservoir, 18 side takeoff separationvessel, 19 piston compressor, 20 membrane unit

In contrast to the known process, the recyle gas downstream of the gasscrubber is not directly compressed, admixed with fresh ethylene andrecirculated to the process, but is first treated according to theinvention In a gas permeation unit Recycle gas compression to higherpressures, which is advantageous for gas permeation, can be achievedusing compressors,

In addition to the main components, approximately 2% by volume ofimpurities (byproducts from synthesis) are still present in the recyclegas, which predominantly consist of the components methane, ethane andvolatile solvents (e.g. methyl chloride). Although these components donot interfere with respect to concentration of the ethylene tocontents >90% by volume, it is advantageous, however, to take intoconsideration these aspects from below which are relevant to allcomponents;

a) The polymer membranes used in the gas permeation can be sensitive tosolvents. Depending on the partial pressure, the temperature and thetype of solvents, there is the risk of a change in performance or arestricted service life.

b) Depending on the permeation behavior of all participating components,these can accumulate in the concentrate (ethylene) or in the permeate.In the case of an enrichment on the concentrate side, the effect of thealtered recyle gas composition on the reaction should be tested.Advantageous measures for reducing this effect are:

ba) the use of higher purity raw materials (e.g. oxygen) or

bb) ejecting enriched components by a side stream, which can be utilizedthermally, for example.

Accumulation of by-products in the permeate is advantageous to achieve aseparation step having high selectivity. Care must be taken to ensurehere that the explosive limits are not exceeded in any stream.

Since the ethylene workup by gas permeation is not prior art, in orderto achieve a statement of the suitability and performance (economicefficiency) of a separation process of this type, simulationcalculations were carried out in the form of a case study on the basisof existing membrane data and permeabilities.

For these studies on the workup of ethylene offgas by gas permeation,the separation task was specified as follows:

a) volumetric flow rate V of the ethylene offgas to be worked up 700m³/h (STP),

b) available working pressure of the compressor p_(F)=7 bar (max. 10bar),

c) temperature θ=30° C.,

d) mean composition of the ethylene offgas in accordance with Table 1(base values),

e) the purpose of separation is an ethylene content in theconcentrate>90% by volume with an ethylene yield >70%.

Three membrane types X, Y, Z were compiled from known gas permeabilties(Table 2) and a numerical simulation of the membrane separation processwas carried out using these.

TABLE 2 Gas permeabilities used (m³/m²hbar) for the numerical simulationMembrane type Component X Y Z Ethylene C₂ H₄ 0.073 0.0135 0.015 Carbondioxide CO₂ 0.43 0.351 0.3 Oxygen O₂ 0.33 0.081 0.083 Argon Ar 0.090.032 0.02* Nitrogen N₂ 0.1 0.0162 0.01 Methane CH₄ 0.1 0.017 0.0143Methyl chloride CH₃Cl 0.14 0.02* 0.016* Selectivity CO₂/C₂H₄ 5.9 26 20*estimated

However, an exact process simulation is only possible with difficulty onthe basis of these data, since exact data are not available in all casesand the relevant permeablities of the individual gas components inmulticomponent mixtures are greatly dependent on the process conditions(e.g. on the partial pressure).

FIG. 7 shows results of the calculations for a single-stage membraneseparation. The diagram shows the purity of the ethylene in theconcentrate as a function of ethylene yield, i.e. as a function of therespective feed stream content of ethylene to be recovered, for threedifferent membrane types. The graphs result from variation of themembrane surfaces. The membrane separation depletes the feed stream(ethylene offgas) in CO₂ as a result of which at the same time theethylene content increases and the resulting concentrate can berecirculated to the process. The CO₂, diffuses through the membrane andpasses Into the permeate. Since the gas treatment by membranes is basedon the relative difference between permeation rates of the participatingcomponents, in addition to the desired CO₂, ethylene also passes intothe permeate.

Depending on the absolute values of the CO₂ and ethylene permeabilitles,and on their ratio to one another (=ideal membrane selectivity),separations of different efficiency thus result. In general, themembrane surface area to be installed and thus the unit size areapproximately inversely proportional to the permeability of a membrane.Furthermore, on account of the finite selectivity of a membrane, withincreasing concentration of a material of value (purity of the ethylenein the concentrate), the loss of material of value (decreasing ethyleneyield in the concentrate) also increases constantly. The quality of themembrane separation therefore, for the same purity of material of value,Increases with increasing yield of material of value, This fact isillustrated for 3 membrane types in FIG. 7.

A further increase in ethylene yield can preferably be achieved bymultistage units. If the material of value arises on the feed side ofthe membrane, as is the case with ethylene enrichment, there is thepossibility to implement a two-stage membrane separation by simpledivision of the membrane surface. FIG. 8 shows the results of atwo-stage membrane separation in a similar manner to FIG. 7, in whichthe membrane surface area of the 2nd stage was kept constant at each ofa number of different values, while the membrane surface area of thefirst stage was varied. The ethylene offgas is concentrated in twosequential stages. While the second stage concentrate represents thestream of material of value, the second stage permeate is recirculatedto a location upstream of the first stage. In the first stage theethylene is enriched to a value below the required purity. The permeateproduced in this stage has significantly lower ethylene contents owingto the membrane separation characteristics. In contrast, the secondstage permeate, which is predominantly formed in the range of highethylene feed concentrations, has comparatively higher ethylene contentsand In consequence contributes to a greater extent to the overall lossesof material of value.

FIG. 8 shows that the ethylene yield can be increased by 5-10% incomparison with a single-stage unit.

The studies carried out show that it is possible to work up the ethyleneoffgas by gas permeation. An ethylene enrichment to >90% by volume canbe achieved at a system pressure of 7 bar even in a single stage with anethylene yield of over 75%. By implementing higher working pressures (to30 bar, for example, using an additional compressor), the gas permeationunit size can be markedly decreased and the performance (selectivity andthus ethylene yield) can be further increased. An increase in yield islikewise possible by implementing a 2-stage unit circuit with permeaterecycling.

The advantages of the process according to the invention are essentiallythat an economic, process-integrated workup of an offgas stream can beperformed using a physical separation process, there is no need forauxiliaries (and thus no interventions into material cycles), and amaterial of value can be recirculated to the process and thus theprocess yield can be increased. By implementing the process according tothe invention, an equally economical and environmentally friendlyincrease in efficiency of the overall process can be achieved.

What is claimed is:
 1. A process for separating off ethylene from a gasmixture comprising more the 8% by volume carbon dioxide or oxygen, whichcomprises separating off ethylene from the gas mixture by gas permeationon the feed side of a membrane selected from the group consisting ofpolymer, palladium and platinum membranes.